The invention concerns methods of supplying a sandwich of polymer films with a linear heat-seal, the apparatus for carrying out and the products resulting from such methods.
A first aspect of the invention concerns the achievement of a high shock-peal-strength, which in particular is needed when the seal is the bottom or top seal of so called heavy duty or industrial bag, which must be constructed to resist the impact of accidental falls which may occur during transport or storage of the filled bag. In industrialised countries it is normal to request that the filled bag must be able to pass drop cycles on each of its faces from at least 2 m height, while in developing countries, where the treatment of the bags often is much rougher, the requirements are often for 4 m drop height.
A distinction is drawn between xe2x80x9cflatxe2x80x9d drops, i.e. drops on one of the two major faces, xe2x80x9cedge dropsxe2x80x9d i.e. drops on one of the two minor faces perpendicular to top and bottom, xe2x80x9ctop dropsxe2x80x9d and xe2x80x9cbottom dropsxe2x80x9d, i.e. drops on to the top and bottom, respectively.
When a filled, heat-sealed bag falls on top or bottom, the shock-action on top or bottom seals is negligible.
xe2x80x9cEdge dropsxe2x80x9d causes a straight peal action on both top and bottom seals.
The panel action by xe2x80x9cflat dropsxe2x80x9d is almost negligible if the bag is a simple xe2x80x9cpillow bagxe2x80x9d without gussets. However the trend in industrial packaging n bags has for many years been towards use of gusseted bags, now more and more practised by xe2x80x9cform-fill-and-sealxe2x80x9d, which process starts with a gusseted tube from reel and in one machine line in sequence makes the bottom seal, cuts the tube into lengths to make open-mouth bags, fills the bag and makes the top seal.
When a gusseted, filled, heat-sealed industrial bag falls xe2x80x9cflatxe2x80x9d and without special precautions being taken by the sealing, there occurs a strong bias type of pealing, which also can be described as tearing, in the four spots where the inner-folds of the gussets interact with the heat-seals. This is because the contents of the bag spread cut horizontally by the impact, thereby tearing in the gussets with forces concentrated around the gussets"" inner-folds.
This means that the locations in the top and bottom seals where these seals intersect with the gusset inner folds are subjected to particularly strong shock-peel or shock-tear forces. The situation is aggravated by the fact that in these locations the seals are relatively weak due to the change from xe2x80x9c2-plyxe2x80x9d to xe2x80x9c4-plyxe2x80x9d bag material.
The conventional way of counter acting this is by two so called xe2x80x9cK-sealsxe2x80x9d in each corner, seals angled at about 45xc2x0 to the bottom and top, starting in the mentioned spots of intersection and bonding each outer-ply to the adjacent ply in the gusset but without bonding gusset-ply to gusset-ply.
Reverting to the straight shock-peeling occurring during xe2x80x9cedge-dropsxe2x80x9d this is most critical in the xe2x80x9c2-plyxe2x80x9d parts of the seal where these border on xe2x80x9c3-plyxe2x80x9d or xe2x80x9c4-plyxe2x80x9d parts, i.e. the longitudinal seam (if such a seam is present) and at the gussets.
When bags accidentally fall, the velocity by which pealing (tearing) takes place will often exceed 5 msxe2x88x921. The standardised laboratory tests for heat-seal-strength are carried out at much lower velocities, and I have found them being without any value for evaluation of the practical performance when a bag falls. For this purpose these tests often give directly misguiding results when different polymer materials or different types of seals are compared. For my comparisons I apply a simplified shock-peel test and a simplified shock-tear test at velocity about 5,5 msxe2x88x921. This test is further explained in the example herein.
When rationally making the (generally linear) top seals and bottom seals in a bag there is always aimed at improved shock-peel strength by promoting swelling through contraction in its plane of the material in the bonded zone and in the immediate adjacent zones of unbonded film material. It is clear that this is needed only on the side of the seal which is predetermined for high shock-peel-strength, i.e. the side adjacent to the contents of the bag. This is conventionally achieved by tapering the edges of the sealing bands or in a similar way making a smooth change between bonded and unbonded zones of the film-sandwich. More precisely, the positive effect of smoothing is that a boundary zone of the heat-seal, which is not bonded, participates in the swelling through contraction in a perpendicular direction to the linear seal. (In my terminology I consider all which has been molten as xe2x80x9csealxe2x80x9d, not limiting this term to the bonded part of the film-sandwich).
However, with the need for downgauging the film materials for bag making, which is a result of ecological and power saving considerations, there has come and will further come a need for much more efficient increase of shock-heat-seal-peel strength. The first industrial realisation of such downgauging has been based on use of stiffer polymer compositions and of higher degrees of malt orientation, in particular highly melt oriented, coextruded films combining HDPE and LLDPE. A later step in such developments, now being introduced by the industry, combines a similar coextrusion technique with cross-lamination (lamination with main directions of orientation criss-crossing) and subsequent biaxial stretching. A survey over inventions involved in this technology is made in the introduction of WO93/14938.
It is clear that the downgauging in itself means reduced shock-peel strength. In addition this strength strongly depends on the stiffness of the material and as mentioned the downgauging requires the use of increased stiffness which reduces the shock-peel strength even more. The higher the stiffness, the lower this strength. A reason for this is that the shock-peel strength depends on the capability in the material to deform elastically and to deform permanently in the xe2x80x9cpeel linexe2x80x9d instead of rupturing, and to undergo such permanent deformation at a sufficient rate. (if the seal falls by shock-peeling, this is normally due to a rupture and not to xe2x80x9cdelaminationxe2x80x9d). Furthermore, the stiffer the film material the lower its capability to take up some of the energy of the shock by elastic elongations in the surroundings of the seal.
Further problems are connected with the orientation, which is an important factor in the downgauging. FIG. 1 illustrates this. The orientation is lost in the seal including its unbonded boundaries. In the bonded part this does not matter, because the thickness has become doubled, but in the unbonded parts the elimination of orientation reduces the shock-strength. (It does not necessarily reduce the strength at lower velocities of peeling, when the material has time to elongate the orient).
A major limiting factor in the downgauging is xe2x80x9cflimsinessxe2x80x9d of the film which makes bag production or the handling of an unfilled bag difficult. In the above mentioned WO93/1428 I disclose how I strongly improve on this by a special cold stretching method, which produces a waved cross-section with thickened top-portions. In the present set of drawings I show this as a microphoto, FIG. 4. (It concerns the film material actually used in example 1). From this it is immediately understood that this structure, which is needed for a strong downgauging, due to the thickness variations also necessitates an improvement of the structure of the seal. (The thickness differences have no significant influence on the general strength properties of the film, since the thinner portions are stretched more strongly).
In experiments preceding the present invention I have tried to improve on the shock-seal-peel strength by use of flat sealing surfaces placed under an angle of 5-15 degrees to each other with the angle opening towards the side where the peal strength is wanted so as to promote the swelling in this side. This gave very improved results when sealing a film-sandwich of a relatively even thickness, but was insufficient or directly harming in the case of significant thickness variations, e.g. at the change around the inner fold of the gusset in a bag. I believe that an explanation tor this is that such sealing makes a border line, which deviates very much from straightness where there are some thickness variations, and that the straightness of this border is a condition for good shock-peal strength.
In GB-A-943457 there is described a method of heat sealing under shrinkage of polymer films according to the introductory portion of claim 1. Heat and pressure are applied by separate sets of heat seal bars to the first and second pressure zone, and the first set of bars must be withdrawn from the sandwich before the second set is applied.
In a new method according to the invention of heat sealing together at least two films of heat shrinkable polymer material, the heat seal being linear and destinied for high-shock-peal-strength from one predetermined side, the two films are subjected to heat, whereby the material in each film contracts in the plane of the film and swells in thickness, and to simultaneous pressure in a squeezed zone so as to produce a heat seal comprising a bonded zone and, on at least the predetermined side, a non-bonded zone in which the film is swollen, in which in an initial stage heat and pressure are applied over an initial pressure zone constituted by a portion of the squeezed zone including the boundary of the squeezed zone located on said predetermined side, and in a second stage heat and pressure are applied over a second pressure zone which overlaps the initial pressure zone and extends from the boundary of said initial pressure zone opposite the boundary of the squeezed zone located on said predetermine side and includes at least a portion of the rest of the squeezed zone adjacent to the said initial pressure zone and pressure in at least a portion of the initial pressure zone located adjacent said squeezed zone boundary is reduced characterised in that heat and pressure are maintained in said overlap zone from the beginning of the initial stage to the and of the second stage.
In a preferred method in a final stage heat and pressure are applied over a final heat and pressure zone which includes the boundary of the squeezed zone opposite said predetermined side and in which heat and pressure are maintained in at least a portion of the squeezed zone throughout the period from the beginning of the initial stage to the end of the final stage.
The second stage may be a final stage, but preferably the final stage does not immediately follow the first stage, that is the second stage is separate from the final stage. There is preferably a continuous progression from the first through the second and final stages.
Preferably the final pressure zone is wider than the initial pressure zone.
Consequently, the first aspect of the present invention concerns a development of the swelling in the critical part of the seal, which at the same time enables the establishment of a sufficiently straight border line between bonded and unbonded zones. It is characterised by the application of sealing members, which by mutual rolling between a pair of such members can change the width of the strip in the film sandwich, which comes under both heat and pressure, hereby first making a first part of said strip, this first part occupying only a fraction of the final width of the seal and being located at the side predetermined for peeling, and subsequently by the mutual rolling extending the width of the strip and releasing the sealing pressure at said predetermined side.
By the first step of the press a relatively straight border line is established, and by the subsequent part of the process the strong swelling is ensured at the same time as the seal is widened and thereby become able to take high shocks in its longitudinal direction, in particular from the shock-tear when the bag falls flat.
In a preferred embodiment of the method the unbonded but heat-treated and swollen zones of the seal at the side predetermined to resist peeling in the final product, are peeled apart while still adhering to the sealing bars and while the material is still molten (referred to hereinafter as hot-peeling).
Said hot-peeling is preferably carried out to an extent to create in the final product an angle of at least 45xc2x0 between the innermost surfaces of the two exterior films of the sandwich in the unbonded but swollen zones where these zones border on the bonded zone.
The effects of the hot-peeling are illustrated especially in FIGS. 3a and b, and it is understandable herefrom that it increases the resistance to cold-peeling in the final product.
Furthermore the first aspect of the invention is preferably carried out in such a way that at the end of the sealing process (in the final stage) the highest sealing pressure is applied on the side of the seal opposite said predetermined side. This promotes the swelling of the seal in the side where this is essential (i.e. the predetermined side), and there may even be squeezed molten material from the side where the thickness of the seal is unessential to the side where it is essential.
The present invention preferably involves passage of the polymer material between a pair of sealing members which apply heat and pressure. At least one of the members has a shape adapted so that by mutual rolling the desired effect is achieved. For instance one of the members may be flat and the other may have a generally angular shape so that, when it is rolled a changing width of the material is subjected to pressure and heat.
More specifically, the first aspect of the invention is preferably carried out in the way that the surface of at least one sealing member is generally wedge-shaped, the band which is under both pressure and heat is started as a band including the top of the wedge and a part of both sides of the wedge, and the mutual rolling taking place over the top of the wedge so that after the rolling the heat and pressure is extended to generally the full width of one side of the wedge, and the pressure is generally released from the other side of the wedge.
The apex of the wedge may be curved or flattened. The rolling preferably taken place about the apex or the centre of curvature of a curved apex.
In this manner the initial stage of heat sealing is started with an angle between the two sealing members opening towards the side where high shock-peel strength is wanted, like in the above mentioned xe2x80x9cpreceding experimentsxe2x80x9d, but in an arrangement which enables an easy adjustment of this angle to obtain for a given film-sandwich the best compromise between straight border line and high swelling.
In order to achieve the above mentioned peeling in the molten state, the polymer composition, sealing temperature and surface of sealing members are preferably adapted to make the surfaces of the film-sandwich stick to the sealing bars also after release of the sealing pressure.
There is preferably made use of auxiliary bars to assist in the hot-peeling action and to ensure release of the sealed film-sandwich from the sealing bars in spite of the intended relatively strong bonding between film material and bars.
As shown in FIG. 2 a practical way of constructing the machinery for the carrying out of the first aspect of the invention is by use of a pair of sealing bars, one sealing bar generally being of wedge-shape, while the other generally is flat and is resilient. The resilience can be achieved in the conventional way shown in this figure, where there is a relatively thin plate of stiff heat and electricity-insulating material (xe2x80x9casbestos substitutexe2x80x9d) between the heater band and the really resilient material (Si-rubber). This as shown enables the sealing of film-sandwiches of strongly variable thickness, especially bag material including gussets and/or longitudinal seam. Alternatively the resilience can under circumstances the achieved by use of a film of reinforced Si-rubber over the heater band. A similar film of reinforced Si-rubber can under circumstances be used on the wedge-formed sealer bar.
A further development of the method according to the first aspect of the present invention is characterised in that after termination of the sealing the sealing members are further mutually rolled over one or more extensions located to the side of the member portion imposing the second pressure zone opposite the side predetermined for peeling, hereby completely releasing the sealing pressure, said extension being kept at a temperature below the temperature needed for sealing and the extensions being adapted to hold the film sandwich during at least a part of the cooling period so as to avoid or reduce the shrinkage of the seal in its longitudinal direction.
The cooling is preferably carried out by blowing cooling air on at least one surface of the seal during the period of holding.
The use of rolling movement of the sealing members relative to each other in connection with the cooling is a second aspect of the present invention and can be exercised independently of the first aspect.
In this further aspect there is provided a method of heat sealing together at least two films of heat shrinkable polymer material in which the two films are subjected to heat, whereby the material in each film contracts in the plane of the film and swells in thickness, and to simultaneous pressure in a squeezed zone so as to produce a heat seal comprising a bonded zone and, on at least one side, a non-bonded zone in which the film is swollen, heat and pressure being applied by sealing members and the heat seal being linear, characterised in that a step of opening the sealing members consists in rolling the bars relative to each other over extensions of said members on one side of the seal, said extensions being kept at a temperature below the minimum heat sealing temperature and the extensions being adapted to hold the film sandwich during at least a part of the cooling period so as to reduce the shrinkage of the seal in its longitudinal direction.
The invention also comprises the products made by the described methods and the apparatus, details of the construction of which appears from the description of the methods.
Thus apparatus according to the invention includes a heat seal station comprising opposed heat seal members, heating means for heating at least one of the heat seal members, activating means for mutually moving the members towards each other whilst heated and for moving the members away from each other, means for feeding a sandwich of at least two polymer films to the heat seal station so that the sandwich is between the heat seal members and means for moving the heat sealed sandwich away form the heat seal station, characterised in that the heat seal members are adapted to apply simultaneous heat and pressure to the film sandwich between initial member zones over an initial pressure zone on the sandwich and to apply simultaneous heat and pressure to the film sandwich over a second pressure zone on the sandwich which overlaps said initial pressure zone between second member zones which overlap said initial member zones include regions of the heat seal members outside but adjacent to the said initial member zones.
Preferably in the apparatus the heat seal members are adapted to apply simultaneous heat and pressure over a final pressure zone on the sandwich between final member zones.
The at least one heat seal member may be oval in shape whereby the members exert heat and pressure over differing widths of the pressure zones as the oval member is rolled relative to the other member. Preferably the at least one heat seal member is wedge-shaped.
The apparatus may use heat seal bars or may use band-sealing. The latter method is normally used for closing of filled bags. The bag stands on a conveyor belt and is continuously passed through the sealing device. This comprised two endless sealer-bands, usually of thin metal or of teflon coated glass fabric, which bands move with the same velocity as the conveyor belt, grip the top of the bag and carry it past one or more heater blocks while the bands are pressed together. The heat is transmitted through one or both sealer-bands into the sack material and performs the sealing. There may subsequently be cooling elements contacting the bands. There may further immediately under these sealer-bands be moving support belts which grip and convey the top of the sack. In this embodiment of the bands may be substantially flat as a conventional band and the other may be flexible and led past a heat block having a slot shaped to provide a profile in the band.
As regards the products the first aspect of the invention specifically concerns a sandwich of film material supplied with a heat-seal being predetermined for high-shock-peel-strength in one side by swelling through contraction of the material in the bonded zone and in the immediate adjacent zones of unbonded film material on said predetermined side of the seal, characterised in that the swelling in said zones of unbonded film material has at least doubled the thickness of the exterior films of the sandwich within a distance from the border of bonding which distance also at least is the double of the thickness of unswollen film, and that there is an angle of at least 45xc2x0 between the innermost surfaces of the two exterior films of the sandwich in the unbonded but swollen zones where these zones border to the bonded zones.
This structure is ideal for shock-peel-strength.
WO89/10312 corresponding to U.S. Pat. No. 5,205,650, issued Apr. 27, 1993, discloses patterns of cold embossment for protection of a seam against shock-actions. The pattern referred to in this publication as xe2x80x9cshock-absorber-bandxe2x80x9d absorbs a part of the shock exercised on the seal during xe2x80x9cedge dropsxe2x80x9d of bags, while the pattern referred to as xe2x80x9cGusset Embossmentxe2x80x9d smooths out the tear forces during flat drops. For optimization of bag drop performance the improvement of the seal itself according to the first aspect of this present invention is preferably combined with such precautions for protection of the seal.
The invention will now be further described with reference to the drawings (to which reference has already been made in the foregoing).
FIG. 1 is a principal sketch of the cross-section of a conventional heat-seal uniting films each of a relatively high degree of molecular orientation. The sketch illustrates a major reason for poor shock-peel-strength when the unbonded parts of the seal have not gained strength by swelling.
FIG. 2 is a sketch of a preferred apparatus for carrying out the sealing according to the invention.
FIG. 3a is a reproduction of a microphotograph in 26 times magnification showing a cross-section of the seal between oriented films in the main part of the seal, i.e. the xe2x80x9c2-plyxe2x80x9d part. In order to enable the reproduction the microphotograph has been retouched while exactly following the structure presented by the photo. The seal is from the process of the example and the figure therefore serves the double purpose of illustrating and documenting the invention.
FIG. 3b is a similar reproduction also from the xe2x80x9c2-plyxe2x80x9d part of the seam and from the process of the example, but the section is cut at about the most critical location of the seam, namely only 1 mm from the intersection with the gusset fold.
FIG. 3c is a similar reproduction and also from the process of the example, but from the gusset or xe2x80x9c4-plyxe2x80x9d part of the seam.
FIG. 4 is a reproduction of a microphotograph in 20 times magnification showing a cross-section of the stretched film used in the example. The purpose of showing this cross-section is for documentation purposes and for explanation of the fact, that FIGS. 3a, 3b and 3c show the films of very different thicknesses also where the films have not been melted and therefore are not swollen.
FIG. 5a, b, c and d are sketches of a band-sealer modified to follow the principle of the first aspect of the invention. While FIG. 5a represents the entire process cycle. FIG. 5b shows section Axe2x80x94A of FIG. 5a and FIGS. 5c and 5d show sections Bxe2x80x94B and Cxe2x80x94C, respectively.